The disclosure relates generally to the field of paving and formulation materials useful for paving surfaces.
Asphalt concretes are sturdy, weather resistant materials that can be used to pave surfaces at relatively low cost. Among the known advantages of asphalt concretes are their ability to support heavy pedestrian and vehicular traffic with substantial durability and wear resistance. Asphalt concretes can also be recycled, in that asphalt concrete removed from existing paved surface can be disintegrated and the resulting pieces (reclaimed asphalt pavement or “RAP”) can be incorporated into fresh asphalt concrete as it is being prepared and laid.
Asphalt concretes are mixtures of one or more bituminous binder materials (“binders”) with one or more aggregate materials (“aggregates”), the mixture typically being composed of about 93%-95% by weight aggregate. Asphalt refers to a highly viscous, tar-like substance of petrochemical origin that is also commonly known as bitumen. Asphalt occurs naturally and is a common product of petrochemical fractionation. Petrochemical asphalt can be supplemented with polymers of various known sorts in order to improve various properties (e.g., one or more of resistance to rutting, thermal cracking, fatigue damage, stripping, and temperature susceptibility) of asphalt concrete which incorporates such polymer-modified asphalts. Use of asphalt as a binder in asphalt concrete paving materials represents the predominant use of asphalt, at least in the United States of America and many other industrialized nations.
A wide variety of materials can be used as aggregate fillers in asphalt concretes. Minerals such as crushed gravels and sands are common aggregates used in manufacture of asphalt concrete paving materials. RAP is also commonly used as an aggregate, RAP being able to contribute both its aggregate fraction and any bituminous binder that remains functional.
Recycling of asphalt-based paving mixtures most commonly occurs in two contexts. First, a common method of recycling (“hot mix recycling”) involves combining RAP with virgin aggregate and new binder (i.e., binder not recovered from the RAP) in a central mixing plant to produce a hot mix-type paving mixtures. In hot mix recycling, RAP can be stored for a period of time prior to its use in a recycled mixture. A second method (“hot in-place recycling”) involves softening an existing asphalt concrete-paved surface by heating it, mechanically removing some or all of the softened pavement, mixing the removed material with additional new binder, virgin aggregate, or both, and repaving the surface using the resulting mixture (i.e., generally without removing the recycled material from the pavement site).
Incorporation of RAP into recycled asphalt concretes is known and has been performed for years. However, use of RAP presents a serious difficulty regarding how the resulting recycled asphalt concrete should be formulated. RAP is derived from previously-produced asphalt concrete and therefore includes both the aggregate and binders and binder-derived products that occur in the previous concrete. Determination of the aggregate composition of RAP tends to be relatively simple, in that any binder or binder-derived products can usually be burned off from a sample of the RAP, leaving substantially only the aggregate. However, determining the amount and utility of binder and binder-derived products in RAP is more complicated.
Asphalt binders (and asphalt concretes including them) are known to oxidize and become stiffer over time as they are exposed to sunlight, temperature changes, and other environmental conditions. In addition to environmental factors, changes in asphalt stiffness and other properties are known to correlate with the initial chemical composition of the asphalt binder, with compounds mixed with the binder (e.g., polymer additives intended to affect concrete properties), and to chemical treatments (e.g., asphalt maintenance procedures and reagents) to which the asphalt is subjected during its use. For at least these reasons, the precise chemical species of binders and binder-products and their relative proportions are not known in RAP. In RAP prepared from freshly-laid asphalt concrete, for example, properties of the binder fraction of the RAP should approximately mirror those of the binders used to form the concrete. However, as the asphalt concrete ages and is subjected to environmental and use conditions, the chemical composition—and hence the properties—of the binder fraction of the RAP can diverge significantly from the composition (and properties) of the original binder. Furthermore, because different portions (e.g., upper surface, interior, and soil-contacting surface) of an asphalt concrete can experience different environmental and use-related stresses, the properties of the binder fraction of RAP can differ within the recycled pavement. Thus, the binder fraction of RAP may exhibit properties which range from the properties of virgin binder to properties of a material that has become so stiff and non-reactive that it acts essentially as an inert “black rock” in an asphalt concrete mixture.
The strength and load-bearing capacity of asphalt concretes are largely attributable to their aggregate components, and these properties of recycled asphalts can be predicted with high reliability based on knowledge of virgin and RAP aggregates. However, the cohesive strength and resistance to degradation and wear are properties that are largely attributable to the binder component(s) of an asphalt concrete. Careful selection and control of binders and their properties is important for formulating practical asphalt concretes.
Difficulties in predicting the properties of the binder component of RAP have led users of RAP to limit the amount of RAP included in recycled asphalt concrete mixtures. A user who over-estimates the degree to which the binder component of RAP acts like virgin binder risks producing asphalt concrete which exhibits low strength and wear resistance. On the other hand, a user who under-estimates the degree to which the binder component of RAP acts like virgin binder risks producing asphalt concrete which is both more expensive than it needs be (asphalt binders and polymer supplements generally being much more expensive than aggregates) and potentially exhibits lower strength and wear resistance than desired.
On account of these shortcomings, practically useful methods of recycling asphalt have generally been limited to those in which not more than about 15% by weight RAP is incorporated into a new asphalt concrete mix. The present disclosure relates to ways of overcoming shortcomings in this field whereby asphalt concrete mixtures having beneficial performance characteristics can be made containing RAP in amounts far in excess of 15% by weight.